First master 12 Volts before tackling high voltage drive systems

Jan. 1, 2018
This article will take a look at some electrical system “basics” and explain how that information can transfer to high voltage system understanding and diagnostics.

High voltage drive systems aren’t really new anymore; after all they’ve been on the market in the United States since late 1999. Shortly after those first hybrid vehicles entered the market, specialized hybrid vehicle training started being offered to the aftermarket as well. Over the years I’ve been fortunate enough to attend classes from numerous providers nationwide, and I was even luckier to be able to develop and run training classes on the subject for technicians, shop owners, and educators all over the country. One lesson that all of those experiences taught me was something that I instinctively already knew as an educator, but sometimes forgot as a technician. That lesson was that unless you have a solid understanding of the “basics” all of the advanced training in the world won’t help you very much. This article will take a look at some electrical system “basics” and explain how that information can transfer to high voltage system understanding and diagnostics. Before you flip the page thinking, “I already know the basics of electricity,” stop and give the subject a chance! While this article will cover some electrical basics they will all be tied into the high voltage system operation and diagnosis and a review of a topic never hurts! 

Series connected NiMh battery modules in a Toyota Prius battery pack

This article will focus on the following electrical “basics” and explain how they relate to the high voltage drive systems: 

  • Ohm’s Law – trust me, this is still important!
  • Watt’s Law
  • Series & Parallel Circuit design
  • Voltage drop
  • Magnetic Induction
  • Counter Electromotive Force (CEMF)

Ohm’s Law and HV Design
As basic as it may seem, Ohm’s Law is the foundation of virtually all electrical diagnosis. No, I’m not implying you’ll be running calculations to figure out the missing part of the equation! What I am saying is that without a firm understanding of the relationship between Ohms, Amperage and Voltage you’ll struggle to understand even the most basic electrical circuit. For instance, have you ever heard a technician say they thought a blown fuse was caused by a corroded (bad) connection? An understanding of Ohm’s Law would have made the technician realize that with a few minor exceptions that simply isn’t a possible reason for the blown fuse. 

So, what is Ohm’s Law? It’s typically written out as “E = I x R”, where “E” represents Voltage (Electromotive Force), “I” represents Amperage (Intensity), and “R” represents Ohm’s (Resistance). Another way to write it is Voltage = Amperage x Resistance. So, what do you really need to know about Ohm’s Law for electrical diagnosis (both 12V and high voltage)? Focus on the relationships. If you have a fixed voltage and resistance goes up (bad connection, corrosion, etc.), then amperage MUST go down (it’s not an option, it’s the LAW). If amperage goes down then you won’t see blown fuses as a result. If on the other hand you have a fixed voltage and resistance goes down, then amperage MUST go up (again, it’s the LAW). In 12V systems, high resistance can cause symptoms like dim lights, while low resistance is likely to result in blown fuses. 

In high voltage systems, high resistance may cause symptoms such as limited power output from the hybrid/electric drive system (caused by lower amperage for the motors). In high voltage systems a “typical” low resistance failure is very unlikely to result in a blown fuse. This can be a little confusing because Ohm’s Law would indicate blown fuses should occur when resistance drops enough. The low resistance does cause an increase in amperage as you’d expect, however the fuses used in high voltage systems are typically “slow blow” fuses. The amperage does increase as it did in the 12V example, however the slow blow fuse doesn’t immediately fail. Instead, the computer system senses the increased amperage and attempts to protect itself from damage. In addition to these self-preservation tactics, the system will set one or more diagnostic trouble codes. So, if you see a high voltage system trouble code stored that references excessive current flow you should immediately think “low resistance.” With that knowledge, you can start testing the related components which are most likely to have a low resistance failure mode such as motor windings.

Toyota Prius battery data PIDS with a low performing module group

Watts – Electrical power
The next logical step after understanding Ohm’s Law is to take a look at Watt’s Law. While Ohm’s Law explains the relationship of Voltage, Amperage, and Resistance; Watt’s Law explains how Voltage and Amperage work together to create Power. Watt’s Law is typically stated as “P = E x I.”  “P” = power in Watts, “E” still represents voltage (Electromotive Force), and “I” still represents the amperage (Intensity). In a typical 12V system this law helps explain why a headlight appears dimmer if there is a high resistance connection. Let’s walk through these two to understand the dim light scenario. First, a high resistance circuit develops due to something like corrosion in a wire or connector. Ohm’s Law dictates this increase in circuit resistance will cause a drop in amperage flowing through the circuit. Watt’s Law would then indicate that a drop in amperage will reduce the watts output from the bulb. As we all know, a lower wattage bulb will put out less light, hence the dimmer appearance of the bulb.

In high voltage systems Watt’s Law doesn’t change. If you want more power from an electric drive system you only have two options. Option #1 — increase the amperage; option #2 — increase the voltage. Increasing amperage in an electric drive system comes with a pretty high cost in terms of weight and “real estate.” Higher amperage needs larger conductors (wires), including those in the motor windings. The extra amount of copper takes more space and increases vehicle weight. That means the seemingly obvious solution to more watts is higher voltage. Unfortunately, if a battery is your electrical source, higher voltage would also require more weight and space. So instead of a larger battery with higher voltage, many systems leverage a boost inverter capable of increasing voltage (and thereby power) under peak loads for limited amounts of time. Think of this as an electrical drive system’s version of a supercharger.

Series vs. Parallel
Since the Watt’s Law examples referenced information related to battery voltage and amperage availability, we should really discuss the difference between series and parallel circuits. For high voltage systems, understanding how each of these circuit types relates to battery configuration is important. In a series circuit there is only a single path for current to flow. Think of this circuit as a one lane road with no shoulders or passing lanes. Any problems along that road will result in a slowdown of the traffic. In electrical terms these road problems would be an increase in resistance, which according to Ohm’s Law, causes a decrease in current flow (the slowdown of the cars on the road). That means the entire circuit is limited in flow by the “weakest” point in the circuit. In 12V system terms think of this as a starter cable where the entire cable appears in good condition, but has one small section that has become mostly severed. That one small section of cable will prevent the starter from working properly even though the rest of the cable is fine. 

Fuel injector waveforms – current ramp and inductive spike

In high voltage systems, the high voltage batteries are made up of numerous low voltage modules wired in series. As those modules are connected end to end (+ to -) the voltage of the assembly increases. Twelve Volt batteries are actually designed using this same model. A typical 12V lead acid battery is actually made up of 6 battery cells, each of which has a resting voltage of approximately 2.1V. When all 6 are wired in series, you end up with the 12.6V lead acid battery that we are used to seeing in automotive starting systems. As we all know from experience, a failure or limitation in any cell within that 12V battery will result in it not operating properly (because of the series configuration). The high voltage battery is constructed in much the same way, although using different battery chemistries. If the hybrid drive system utilizes a NiMh (Nickel Metal Hydride) each cell will have a nominal rating of 1.2V. In a configuration such as a Toyota Prius there are 6 cells wired in series within each battery module. This results in 7.2V battery modules, which are also connected in series until the desired battery voltage for the drive system is achieved. The 2010 vintage “standard” Prius model for instance connects 28 of those 7.2V modules in series to create a 201.6V battery. Just like in the 12V system however, the series configuration means the entire battery pack is only as good as the single weakest cell within the pack. In this case there are 168 individual 1.2V NiMh cells you are depending on. 

Example of motor windings (inductor) in High Voltage drive systems

Parallel circuits are very different from series circuits. Instead of a single path for current to flow like a series circuit has, a parallel circuit has at least two separate current paths. That means if one path fails, the other can still work independently. Let’s relate this back to the road analogy I used in the series circuit discussion. A parallel circuit now has multiple lanes running in the same directly. This means you can now get more cars through at a time (lower resistance) and if one lane has an issue the others are still operational. So, while a single point failure will result in a decrease in flow, it won’t totally prevent flow like a failure in a series circuit would. In battery terms, a parallel connection works much differently as well. If two 12V batteries are wired in parallel (both + terminals connected together, and both – terminals connected together) like they are in light duty diesel engine starting systems, the system voltage is still 12V. Wiring batteries in parallel doesn’t increase the voltage, but it does increase the available amperage. This means if you connect two high voltage batteries in parallel you can do things like increase the available current to the electric drive system, or more likely, increase the available amp hours for the system (the latter is more likely to be seen in PHEV and EV applications than in traditional hybrids to increase the available range). The Tesla Model S for instance uses a combination of series and parallel configurations for the battery pack to build the voltage to the desired level (series connections), and then increase the available amp hours (parallel connections) to increase the range.        

Voltage drop             
To further explain high voltage battery issues and monitoring, you need to understand voltage drops. In my opinion, voltage drop testing is one of the most important electrical tests every technician should be proficient in doing. I typically refer to this as where voltage is “used” when current is flowing. In simple terms, a 12V battery has 12 volts of “push” available at one terminal which must be completely “used” (dropped) by the time it returns to the opposite terminal. If a circuit only has one load (a single bulb in series for instance) then virtually all of that 12V should be used (dropped) by that single load. If voltage is being “used” (or dropped) elsewhere in the circuit there is less voltage available for the load. Think back to the example of the dim headlight earlier in this article. In that example I mentioned the high resistance circuit causing a decrease in the circuit amperage. What I left out however was the voltage drop associated with that high resistance circuit. Any time there is additional (undesired) resistance in a circuit it will cause a drop in the voltage available to be “used” by the intended load of the circuit. That means in the light bulb example, if the high resistance wire or connector “uses” (drops) 2V, the light bulb can only “use” 10V (instead of the 12V it was designed to “use”). In terms of high voltage batteries, any undesired resistance within the pack itself (internal within the cells, bus bar connections, etc.) will cause a voltage drop within the pack whenever current is flowing.  Battery packs utilize sensor connections to monitors sections of the pack for these drops. The sections of the packs are typically referred to as Voltage Blocks (or V-Blocks) and any excessive variance within that pack is likely to set a battery pack imbalance code and restrict performance to the capabilities of the weakest section of the pack. 

3 Phase motor control with aggressive switching caused by a shorted stator

Magnetic induction
So, even if that battery is working correctly, how can you get more power from the battery to the high voltage drive motor? As mentioned earlier in the article, to increase the available power the preferred option is to increase the voltage. One way manufacturers increase this voltage above what the battery pack supplies is to use a boost inverter. This utilizes the same magnetic induction (self-induction) concept as the primary coil of an ignition coil, or the voltage spike seen at turn off when scoping a fuel injector voltage waveform. When current flows through a coil of wire a magnetic field is created. When that field collapses across the coil it induces a voltage “spike” into the coil, and the coil becomes a voltage source. In traditional 12V ignition systems the primary coil spike isn’t utilized (in fact it has to be managed to prevent damage) but it can be seen if using an oscilloscope to monitor the ignition primary voltage triggering. In high voltage boost inverters, the boost reactor coil is wired in series between the battery and the DC bus circuit in the inverter assembly. To increase the available voltage, current through that reactor coil is increased (higher current draw from the battery pack and/or generator). The increase in current causes an increased magnetic field strength surrounding the reactor coil. When that current is decreased again (but not fully shut off), the magnetic field partially collapses across the coil inducing a voltage “spike.” If the switching is done at a high enough frequency (and filtered through the control circuitry) the output appears as an increased DC bus voltage available for the motor drive. Because the boost reactor needs an increase in current to function, a battery pack that is current limited (increased internal resistance or decreased capacity) will limit the amount of boost available. This could be seen as a decrease in the acceleration performance of the vehicle. 

3 Phase motor control with balanced switching

CEMF
Lastly, let’s take a look at the concept of Counter Electromotive Force (CEMF). This simply states that any current flowing through an inductor (a coil wire) will create a voltage within that inductor that “pushes” back against the initial voltage source. The cause of that “push” back is the expansion of the magnetic field building around the coil as current begins to flow. If you do current ramping of 12V system components such as ignition coils or fuel injectors you are already familiar with this concept. In fact, it’s the CEMF that causes the current to ramp up instead of climbing quickly. The reason we see a decrease in ramping when a coil or injector shorts is because the shorted coil develops a weaker magnetic field, which in turn results in less CEMF. This same concept holds true with the windings in a high voltage drive motor. The main difference in a shorted high voltage drive motor is that the current flowing through the motor windings (coils) is continually being monitored. If the controller senses the rate of current increase is occurring to quickly it will still attempt to regulate the drive motor supply but the switching from the motor controller will have to be modified to do so. This results in a more aggressive switching of the current going from the inverter to the electric drive motor because the current isn’t ramping up correctly. Typically that will result in diagnostic trouble codes related to a “short.” The aggressive switching can be seen using an oscilloscope equipped with the right current monitoring clamps to help confirm the diagnostic codes but, if it’s gotten to that point the electric machine (motor/generator) likely will need to be replaced and it’s possible the inverter will have suffered damage as well.  Analysis of the current waveform may allow you to confirm the ability of the inverter to operate correctly for the time being if a new electric machine is installed, however the long term potential damage caused by the aggressive switching will be impossible to know.

This article really just scratches the surface of the connection between “basic” electrical principles and the high voltage drive systems. Overall if you take time to really understand electrical basics you’ll have a much easier time picking up the “advanced” content as it relates to not only hybrid/electric drive systems but also for drivability-related diagnostics on the base engine system.  

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